Mitigation of Particulates and NOx in Engine Exhaust

ABSTRACT

An emission treatment system and associated method for treating an exhaust stream containing nitrogen oxides and particulate matter are disclosed. One embodiment of the system comprises a flow-through oxidation catalyst, a reductant injector downstream from the oxidation catalyst, a particulate filter downstream from the reductant injector, an SCR catalyst downstream from the particulate filter and an ammonia oxidation catalyst downstream from the SCR catalyst. An embodiment of the method comprises passing the exhaust gas stream through the oxidation catalyst, injecting a reductant into the exhaust gas stream, passing the exhaust gas stream through the particulate filter, passing the exhaust gas stream through an SCR catalyst, and passing the exhaust gas stream through an ammonia oxidation catalyst.

FIELD OF THE INVENTION

Embodiments of the invention relate generally to exhaust treatmentsystems and methods. More particularly, embodiments of the presentinvention pertain to exhaust treatment systems and methods thatefficiently mitigate both nitrogen oxides and particulate matter inemissions.

BACKGROUND

Both diesel engines and gasoline engines that run lean producesignificant amounts of particulates in addition to NOx. Compressionignition diesel engines have great utility and advantage as vehiclepower trains because of their inherent fuel economy and high torque atlow speed. Diesel engines run at a high air to fuel (“A/F”) ratio undervery lean fuel conditions. Because of this, they have very low emissionsof gas phase hydrocarbons and carbon monoxide. However, diesel exhaustis characterized by relatively high concentrations of nitrogen oxides(“NOx”) and particulates. Diesel engine exhaust is a heterogeneousmixture which contains not only gaseous emissions such as carbonmonoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides, butalso condensed-phase materials (liquids and solids) which constitute theso-called particulates or particulate matter. Emissions treatmentsystems for diesel engines must treat all of the components of theexhaust to meet emissions standards set by various regulatory agenciesthroughout the world.

The total particulate matter emissions of diesel exhaust contain threemain components. One component is the solid, dry, solid carbonaceousfraction or soot fraction. This dry carbonaceous fraction contributes tothe visible soot emissions commonly associated with diesel exhaust. Asecond component of the particulate matter is the soluble organicfraction (“SOF”). The SOF can exist in diesel exhaust either as a vaporor as an aerosol (fine droplets of liquid condensate) depending on thetemperature of the diesel exhaust. It is generally present as condensedliquids at the standard particulate collection temperature of 52° C. indiluted exhaust, as prescribed by a standard measurement test such asthe U.S. Heavy Duty Transient Federal Test Procedure. These liquidsarise from two sources: (1) lubricating oil swept from the cylinderwalls of the engine each time the pistons go up and down; and (2)unburned or partially burned diesel fuel. The third component of theparticulate matter is the so-called sulfate fraction, which is formedfrom small quantities of sulfur components present in the diesel fuel.

Catalyst compositions and substrates on which the compositions aredisposed are typically provided in diesel engine exhaust systems toconvert certain or all of these exhaust components to innocuouscomponents. For instance, oxidation catalysts, which may be referred toas diesel oxidation catalysts (“DOCs”), containing platinum groupmetals, base metals and combinations thereof, facilitate the treatmentof diesel engine exhaust by promoting the conversion of both unburned HCand CO gaseous pollutants, and some proportion of the particulate matterthrough oxidation of these pollutants to carbon dioxide and water. Suchcatalysts have generally been disposed on various substrates (e.g.,honeycomb flow through monolith substrates), which are placed in theexhaust of diesel engines to treat the exhaust before it vents to theatmosphere. Certain oxidation catalysts also promote the oxidation of NOto NO₂.

In addition to the use of oxidation catalysts, particulate filters areused to achieve high particulate matter reduction in diesel emissionstreatment systems. Known filter structures that remove particulatematter from diesel exhaust include honeycomb wall flow filters, wound orpacked fiber filters, open cell foams, sintered metal filters, etc.However, ceramic wall flow filters, described below, receive the mostattention. These filters are capable of removing over 90% of theparticulate material from diesel exhaust.

Typical ceramic wall flow filter substrates are composed of refractorymaterials such as cordierite or silicon-carbide. Wall flow substratesare particularly useful to filter particulate matter from diesel engineexhaust gases. A common construction is a multi-passage honeycombstructure having the ends of alternate passages on the inlet and outletsides plugged. This construction results in a checkerboard-type patternon either end of the honeycomb structure. Passages plugged on the inletaxial end are open on the outlet axial end. This permits the exhaust gaswith the entrained particulate matter to enter the open inlet passages,flow through the porous internal walls and exit through the channelshaving open outlet axial ends. The particulate matter is therebyfiltered on to the internal walls of the substrate. The gas pressureforces the exhaust gas through the porous structural walls into thechannels closed at the upstream axial end and open at the downstreamaxial end. The accumulating particles will increase the back pressurefrom the filter on the engine. Thus, the accumulating particles have tobe continuously or periodically burned out of the filter to maintain anacceptable back pressure.

Catalyst compositions deposited along the internal walls of the wallflow substrate assist in the regeneration of the filter substrates bypromoting the combustion of the accumulated particulate matter. Thecombustion of the accumulated particulate matter restores acceptableback pressures within the exhaust system. Soot combustion can be passive(e.g., with catalyst on the wall flow filter and adequately high exhausttemperatures), though for many applications active soot combustion isalso required (e.g., production of a high temperature exotherm in theexhaust up-stream of the filter). Both processes utilize an oxidant suchas O₂ or NO₂ to combust the particulate matter. During activeregeneration, CO is sometime generated by the regeneration process.

Passive regeneration processes combust the particulate matter attemperatures within the normal operating range of the diesel exhaustsystem. Preferably, the oxidant used in the regeneration process is NO₂since the soot fraction combusts at much lower temperatures than thoseneeded when O₂ serves as the oxidant. While O₂ is readily available fromthe atmosphere, NO₂ can be generated through the use of upstreamoxidation catalysts to oxidize NO in the exhaust stream.

In spite of the presence of the catalyst compositions and provisions forusing NO₂ as the oxidant, active regeneration processes are generallyneeded to clear out the accumulated particulate matter, and restoreacceptable back pressures within the filter. The soot fraction of theparticulate matter generally requires temperatures in excess of 500° C.to burn under oxygen-rich (lean) conditions, which are highertemperatures than those typically present in diesel exhaust. Activeregeneration processes are normally initiated by altering the enginemanagement to raise temperatures in front of the filter up to 570-630°C. One common way that has been developed to accomplish activeregeneration is the introduction of a combustible material (e.g., dieselfuel) into the exhaust and burning it across a flow-thru DOC mountedupstream of the filter. The exotherm from this auxiliary combustionprovides the sensible heat (e.g. about 550-700° C.) needed to burn sootfrom the filter in a short period of time (e.g. about 2-20 min.).Depending on driving mode, high exotherms can occur inside the filterwhen the cooling during regeneration is not sufficient (low speed/lowload or idle driving mode). Such exotherms may exceed 800° C. or morewithin the filter.

Current particulate filter systems that include the capability foractive filter regeneration (soot combustion) under high temperatureconditions (e.g., about 550-650° C.) typically consist of a light-offDOC upstream of a downstream particulate filter, with a selectivecatalytic reduction (“SCR”) catalyst disposed between the DOC and theparticulate filter. However, an emission system that reduces bothparticulates and NOx can have several other configurations. For example,there are systems in which the NOx removal catalyst (e.g., SCR, LNT orLNC) is upstream as a separate device from the particulate filter. Inother systems, the NOx removal catalyst is placed downstream as aseparate device. Another configuration involves integration of the NOxcatalyst and particulate removal, i.e. SCR, LNT, or LNC on a particulatefilter.

Certain conventional coating designs for wall flow substrates have ahomogeneous distribution of catalyst along the entire axial length ofthe internal walls. In such designs the platinum group metalconcentration is typically adjusted to meet the emissions requirementsunder the most stringent conditions. Most often, such conditions referto the catalyst's performance after the catalyst has aged. The costassociated with the required platinum group metal concentration is oftenhigher than is desired.

Other conventional coating designs for wall flow substrates employconcentration gradients of catalyst along the axial length of thesubstrate. In these designs, certain catalyst zones (e.g., upstreamzones) have a higher concentration of platinum group metals than doadjacent axial zones (e.g., downstream zones). Typically, the internalwalls of the axial zone where the higher amount of catalyst is disposedwill have a lower permeability than an adjacent zone having a lowerwashcoat loading. A gas stream passing along the length of the inletpassage will preferentially travel through the internal wall in thesegments that have the highest permeability. Thus, the gas stream willtend to flow through the internal wall segments that have lower amountsof washcoat. This differential flow pattern can result in inadequatepollutant conversion. For instance, certain gaseous pollutants, e.g.,unburned saturated hydrocarbons, require contact with higherconcentrations of platinum group metal components than do unsaturatedhydrocarbons to achieve sufficient levels of combustion. Thisrequirement is exacerbated during operating conditions where the exhausttemperatures are cooler, e.g., at startup.

A typical emission treatment system is shown in FIG. 1. The exhaust gasstream emitted from a diesel engine 10 travels through exhaust gasconduit 11. A hydrocarbon (“HC”) injector 24 is provided downstream fromthe engine. The HC injector 24 can inject numerous compounds containingcarbon and hydrogen upstream from oxidation catalyst 12 that mix withthe exhaust gas stream from the engine 10. The oxidation catalyst 12,which in a diesel exhaust system is a diesel oxidation catalyst (“DOC”)on a flow-through substrate, is located downstream from the HC injector24. The exhaust gas stream passes through the oxidation catalyst 12,which promotes the oxidation of its constituent compounds and down to asoot or particulate filter 16. A reductant injector 14 is positioneddownstream from the particulate filter 16 upstream from SCR catalyst 18.A mixer 17 is positioned upstream from the SCR catalyst to mix injectedreductant. The system may further include an ammonia oxidation catalyst20.

As can be appreciated from the above, current systems pose a number ofissues concerning size, in particular, the length of the system toaccommodate the need for adequate mixing. Longer systems result in moreheat loss, which is not good for catalysts efficiency. In addition, inSCR systems that inject urea or other reductants into the system, themixer 17 is required to adequately mix the reductant. Mixers createturbulence, and turbulence leads to pressure drop. High backpressureadversely affects engine operation. Furthermore, in existing systems,not all of the urea decomposes to ammonia especially at lowertemperatures, for example, below 300° C. Accordingly, it would bedesirable to provide engine exhaust treatment systems and methods thatalleviate one or more of these issues.

SUMMARY

According to an embodiment of the invention, an emission treatmentsystem for treating an exhaust stream containing NO_(x) and particulatematter is provided, comprising a flow-through oxidation catalyst, areductant injector for introducing a reductant into the exhaust streamlocated downstream from the oxidation catalyst, a particulate filterthat does not contain an SCR catalyst for removing the particulatematter from the exhaust stream located downstream from the reductantinjector, an SCR catalyst for promoting chemical reactions between thenitrogen oxides and the reductant located downstream from theparticulate filter, the particulate filter being effective to dispersethe reductant prior to the reductant contacting the SCR catalyst. In oneembodiment, an optional ammonia oxidation catalyst is located downstreamfrom the SCR catalyst. Various reductants, including urea, can beutilized. In one embodiment, the particulate filter is uncatalyzed. Analternative embodiment provides a particulate filter that is catalyzed.In certain embodiments, the ammonia oxidation catalyst and a COoxidation catalyst can be combined onto the same substrate and locateddownstream of the SCR catalyst. The function of the CO oxidationcatalyst is to provide oxidation of CO generated during activeregeneration.

In one or more embodiments, the particulate filter and the SCR catalystare closely coupled. According to one embodiment, the particulate filterand the SCR catalyst are positioned inside a single canister. Accordingto one or more embodiments, the reductant is urea and the particulatefilter is configured so that substantially all of the urea that exitsthe particulate filter is converted to ammonia and isocyanic acid.

Additional embodiments of the emission treatment system include ahydrocarbon injector for introducing hydrocarbon compounds into theexhaust stream located upstream of the oxidation catalyst and a meteringsystem in fluid communication with the reductant injector. In certainembodiments of the invention, the SCR catalyst and the ammonia oxidationcatalyst are located on the same substrate.

The particulate filter in at least one embodiment has a first catalystzone for creating NO₂ from the oxidation of NO. According to oneembodiment, the catalyst zone extends for at least a portion of theaxial length of the filter. In other embodiments, the particulate filteralso includes a second catalyst zone for the oxidation of CO.Furthermore, in certain embodiments of the invention, the particulatefilter also comprises a urea hydrolysis catalyst and/or a base metaloxide for burning soluble organic fraction of soot captured in thefilter.

According to some embodiments of the invention, a method of treating anexhaust gas stream containing nitrogen oxides and particulate matter isprovided. The method comprises passing the exhaust gas stream containinghydrocarbon compounds through a oxidation catalyst, injecting areductant into the exhaust gas stream downstream from the oxidationcatalyst and upstream of a particulate filter, passing the exhaust gasstream through the particulate filter to cause the reductant to mix withthe exhaust gas stream, and passing the exhaust gas stream exiting theparticulate filter through an SCR catalyst. In one embodiment, themethod may optionally include passing the exhaust gas stream exiting theSCR catalyst through an ammonia oxidation catalyst. In anotherembodiment, the method optionally includes injecting hydrocarboncompounds into the exhaust gas stream upstream of the oxidationcatalyst. In certain embodiments, injecting hydrocarbon compounds intothe exhaust gas stream is regulated by a first control system.Additional embodiments employ a second control system to regulate theinjecting of a reductant into the exhaust gas stream. The hydrocarbonsfrom the first injector will be oxidized and generate and exotherm, andthe heat generated could be used to heat the system quickly duringstart-up. Another other use of this heat is for active regeneration. Inone embodiment, hydrocarbon is injected in the second injector and ahydrocarbon SCR catalyst is located downstream of the filter.

In some embodiments of the method, passing the exhaust gas streamthrough the particulate filter comprises contacting the exhaust gasstream with a base metal oxide catalyst for burning soluble organicfractions or with a urea hydrolysis catalyst on the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

FIG. 1 is a schematic illustration of an emission treatment system inaccordance with the prior art; and

FIG. 2 is a schematic illustration of a of an emission treatment systemin accordance with one embodiment of the invention.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

An embodiment of the inventive emission treatment system is shown inFIG. 2. The exhaust gas stream emitted from a diesel engine 110 travelsthrough exhaust gas conduit 111. In one embodiment, an optionalhydrocarbon (“HC”) injector 124, which can assume variousconfigurations, including that of a nozzle, is provided downstream fromthe engine. The HC injector 124 can inject numerous compounds containingcarbon and hydrogen upstream from oxidation catalyst 112 that mix withthe exhaust gas stream from the engine 110. A suitable control systemselected from those known in the art can be employed to regulate the HCinjection process For example, a closed feedback loop can be configuredto operated based on the desired temperature behind the oxidationcatalyst, A higher desired temperature can be achieved by increasing theamount of hydrocarbon injected. A temperature sensor can be utilized todetermine whether the minimum temperature for injection is met. Theoxidation catalyst 112, which in a diesel exhaust system is a dieseloxidation catalyst (“DOC”) on a flow-through substrate is locateddownstream from the HC injector 124.

The exhaust gas stream passes through the oxidation catalyst 112, whichpromotes the oxidation of its constituent compounds and down to a sootor particulate filter 116. A reductant injector 114 is positionedupstream of the particulate filter 116. The filter 116 does not containan SCR catalyst The system may further include an optional ammoniaoxidation catalyst 120. The reductant injector 114 introduces aqueousurea into the exhaust gas stream. Water quickly evaporates from the ureato yield gaseous urea:

(NH₂)₂CO (aq)→(NH₂)₂CO (g)+H₂O (g)

The urea then decomposes to ammonia in two steps, the first of which isthermolysis of urea and the second of which involves hydrolysis ofisocyanic acid:

(NH₂)₂CO→NH₃(g)+LNCO(g)

HNCO (g)+H₂O(g)→NH₃ (g)+CO₂(g)

The gas stream exiting the oxidation catalyst is passed throughparticulate filter 116. Nearly all of the ammonia formed by thedecomposition of urea passes through the filter 116. Unreacted ureaaccumulates on the large surface area of the filter 116. According toone or more embodiments of the present invention, the filter 116prevents unreacted urea from traveling any further downstream. Unreactedurea may have a tendency to deactivate the SCR catalyst. Anotherfunction served by the filter 116 is to facilitate the mixing of ammoniaand urea with the exhaust gas without increasing backpressure in thesystem. An introduction port or valve can be used to meter preciseamounts of the reductant. A suitable control system can be utilized toregulate the reductant injection process. Examples of control systemsinclude a NOx sensor upstream of the injector (open loop control); a NOxsensor downstream of the SCR catalyst (closed loop control); predictivecontrol based on engine conditions; and combinations of these systems.

The filter 116, which can be either catalyzed with a NOx oxidationcatalyst or uncatalyzed, facilitates the mixing of the reductant and theexhaust gas stream. If the filter is catalyzed, a catalyst that does notcause the oxidation of ammonia should be utilized. SCR catalyst 118 ispresent downstream from the filter 116. In certain embodiments there isa gap 130 between the filter 116 and the SCR catalyst 118. In addition,certain embodiments of the filter 116 comprise a base metal oxide forburning soluble organic fraction of soot captured therein. Otherembodiments comprise a urea hydrolysis catalyst. Optionally, the filter116 and the SCR catalyst 118 can be closely coupled. For example, asdepicted in FIG. 2, they can be positioned in adjacent “bricks” inside asingle canister. A “brick” of material, such as cordierite or the like,is a portion of a honeycomb-type carrier member having a plurality offine gas-flow passages extending from the front portion to the rearportion of the carrier member. Downstream from the SCR catalyst 118, theexhaust gas stream passes through an optional ammonia oxidation catalyst120. The SCR catalyst 118 can be located on the same substrate as theammonia oxidation catalyst 120.

Wall Flow Substrates

Wall flow substrates useful for filtration of particulate matter fromexhaust streams have a plurality of fine, substantially parallel gasflow passages extending along the longitudinal axis of the substrate.Typically, each passage is blocked at one end of the substrate body,with alternate passages blocked at opposite end-faces. Such monolithiccarriers may contain up to about 400 or more flow passages (or “cells”)per square inch of cross section, although far fewer may be used. Forexample, the carrier may have from about 100 to 350, cells per squareinch (“cpsi”). Wall flow substrates typically have a wall thicknessbetween about 0.012 to 0.020 inches. Examples of suitable wall flowsubstrates have a wall thickness of between 0.012 and 0.015 inches. Anexemplary aspect ratio for a filter is 0.75 to about 1.5.

Suitable wall flow filter substrates are composed of ceramic-likematerials such as cordierite, {acute over (α)}-alumina, silicon carbide,silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia,alumina-titanate, or zirconium silicate, or of porous, refractory metal.Wall flow substrates may also be formed of ceramic fiber compositematerials. Examples of suitable wall flow substrates are formed fromcordierite, alumina-titanate, and silicon carbide. Such materials areable to withstand the environment, particularly high temperatures,encountered in treating the exhaust streams.

Suitable wall flow substrates for use in the inventive system includethin porous walled honeycombs (monoliths) through which the fluid streampasses without causing too great an increase in back pressure orpressure across the article. Normally, the presence of a clean wall flowarticle will create a back pressure of I inch water column to 10 psig.According to embodiments of the invention, ceramic wall flow substratesused in the system are formed of a material having a porosity of atleast 40% (e.g., from 50 to 75%) having a mean pore size of at least 5microns (e.g., from 5 to 30 microns). In certain embodiments, thesubstrates have a porosity of at least 55% and have a mean pore size ofat least 10 microns. The porous wall flow filter used according toembodiments of the invention can be catalyzed in that the wall of saidelement has thereon or contained therein one or more catalyticmaterials. Catalytic materials may be present on the inlet side of theelement wall alone, the outlet side alone, both the inlet and outletsides, or the wall itself may consist all, or in part, of the catalyticmaterial. After coating with catalyst, the substrates are driedtypically at about 100° C. and calcined at a higher temperature (e.g.,300 to 450° C.). After calcining, the catalyst loading can determined bethrough calculation of the coated and uncoated weights of the substrate.As will be apparent to those of skill in the art, the catalyst, loadingcan be modified by altering the solids content of the coating slurry.Alternatively, repeated immersions of the substrate in the coatingslurry can be conducted, followed by removal of the excess slurry asdescribed above.

DOC Catalyst Compositions

The oxidation catalyst formed on the particulate filter can be formedfrom any composition that provides effective combustion of unburnedgaseous and non-volatile hydrocarbons (i.e., the VOF) and carbonmonoxide. In addition, the oxidation catalyst should be effective toconvert a substantial proportion of the NO of the NOx component to NO₂.As used herein, the term “substantial conversion of NO of the NOxcomponent to NO₂” means at least 20%, and preferably between 30 and 60%.Catalyst compositions having these properties are known in the art, andinclude platinum group metal- and base metal-based compositions. Anexample of oxidation catalyst composition that may be used in theemission treatment system contains a platinum group component (e.g.,platinum, palladium or rhodium components) dispersed on a high surfacearea, refractory oxide support (e.g., γ-alumina) which is combined witha zeolite component (for example, a beta zeolite). A suitable platinumgroup metal component is platinum.

Platinum group metal (“PGM”) based compositions suitable for use informing the oxidation catalyst may have a mixture of platinum,palladium, rhodium, and ruthenium and an alkaline earth metal oxide suchas magnesium oxide, calcium oxide, strontium oxide, or barium oxide withan atomic ratio between the platinum group metal and the alkaline earthmetal of about 1:250 to about 1:1, and particularly about 1:60 to about1:6. Catalyst compositions suitable for the oxidation catalyst may alsobe formed using base metals as catalytic agents.

The catalyst loading in the DOC can be varied to between about 40 g/ft³and 100 g/ft³. In specific embodiments, the catalyst is can be chosenfrom Pt and/or Pd, both of which are good oxidation catalysts forhydrocarbons. The current price of platinum is much higher than forpalladium, thus the latter offers the advantage of reduced cost;however, this may change in the future depending on catalyst demand.Platinum is very active for hydrocarbon oxidation reactions and israther resistant to poisoning. Palladium can be less active and issusceptible to poisoning, e.g. by sulfur. However, under lean exhaustconditions and temperatures that might exceed 800° C., platinum canexperience thermal sintering and thereby reduction in oxidationactivity. Addition of palladium and its interaction with the platinumresults in a substantial reduction in the high temperature sintering ofthe platinum and thereby maintenance of its oxidation activity. If thetemperatures of exposure are kept low, Pt-only may be a good option toobtain the highest possible oxidation activity. However, inconfigurations in which high temperatures (e.g. 800° C.) areanticipated, especially internal to the filter, inclusion of some Pd isdesired. Pt:Pd ratios to obtain acceptable Pt stability with the highestoxidation activity are between about 10:1 and 4:1; however, ratios aslow as 2:1 and 1:1 are also within the scope of the invention. Higher Pdcontents (e.g., 1:2) need to be are also within the scope of the presentinvention. In certain embodiments, Pd with no platinum may be used.

The catalyst is dispersed on a suitable support material such as arefractory oxide with high surface area and good thermal stability suchas a high surface area aluminum oxide. Suitable aluminas includealuminas stabilized with lanthana, for example 4 wt. % lanthana. Mixtureof such aluminas in a 50:50 wt blend can be utilized as a suitablesupport material]. Other aluminas that are doped or treated with oxidessuch as SiO₂, ZrO₂, TiO₂, etc.) to provide stabilization or improvedsurface chemistries can also be utilized. Other suitable supportmaterials, include, but are not limited to, ZrO₂ and TiO₂ can be used.In addition to the catalyst support oxides discussed above, it mightprove useful to include other catalytically functional oxides toincorporate into the catalytic zone. Examples of these include CeO₂,Pr₆O₁₁, V₂O₅, and MnO₂ and combinations thereof and solid solution oxidemixtures, etc. These oxides can contribute to burning of hydrocarbons,especially heavy fuel derived hydrocarbons, and deposited coke/sootderived from disproportination of the injected fuel and in this way giveadditional combustion activity to the catalytic zone, plus preventdeactivation of the catalyst by the deposition hydrocarbon derived coke.

Filter Catalyst Coating

The loading of the oxidation catalyst in the zone on the filtersubstrate is typically is limited to control the contribution of thephysical volume of the catalyst coating filling the pore volume of thefilter substrate and thereby adversely affecting the flow resistancethrough the filter wall and thus the back-pressure. On the other hand,with high loadings of catalyst on the support oxide we have to providesufficient surface area for good catalyst dispersion. As an example, acatalyst loading on the inlet zone of about 60 g/ft³, a dry gain (DG) of0.5 g/in³ in the zone is acceptable. The DG can be adjusted taking intoconsideration the optimum catalyst loading, alumina to other (denser)oxide weight ratio, and other factors.

The ratio of the zone length/volume to total filter length/volume canvary between about 0.20 to 1.0, for example, this value can be 0.25, 0.5or 0.75. Thus, for example, an 11.25″ diameter×14.0″ long filtersubstrate a zone length/depth of ca. 3.0″ could be used or a ratio of0.21 of total length/volume of the filter. However, determination of themost effective zone length/volume ratio will be part of catalyzed filteroptimization for a particular exhaust system design.

NOx Reduction Catalysts

For most US heavy duty diesel applications, starting in 2007, the aftertreatment system utilizes engine design and calibration. However, in theUnited States, particularly starting in 2010, stricter NOx emissionsstandards are not expected to be met by engine design and calibrationmeasures alone and a NOx reduction after treatment catalyst will berequired. The NOx reducing catalyst according to one or more embodimentsof the invention can comprise an SCR catalyst, a lean NOx catalyst, alean NOx trap (LNT), or a combination of these.

It should be noted that the engine-out NOx is mainly in the form of NOwith low levels of NO₂ and that the catalyst loadings and ratiosemployed in the zone and body of the zoned particulate filter can betailored to control the level of filter-out NO₂ versus NO. The oxidationreaction, represented by NO+½O₂—>NO₂, can be controlled by the catalystfunction. The effectiveness of the down-stream SCR or LNT can beenhanced by control of the NO₂/NO ratio.

For an SCR reaction, there three reaction conditions can be considereddepending on the NO₂/NO ratio:

(1) Standard: 4 NH₃+4 NO +O₂—>4 N₂+6H₂O

(2) “Fast”: 4 NH₃+2 NO +2 NO₂—>4 N₂+6H₂O

(3) “Slow”: 4 NH₃+3 NO₂—>3.5 N₂+6H₂O.

From the above three conditions, it can be seen that the desired “fast”or more efficient SCR reaction occurs if the NO₂ to NO ratio is 1:1 andrelative to engine-out it is expected to require an oxidation functionto increase the relative amount of NO₂. According to embodiments of theinvention, the catalyst on the zoned particulate filter can contributeto this function and tailoring the catalyst loading and ratio can beused to achieve this. It is believed that the 1:1 ratio will give thebest down-stream SCR reaction. Higher levels of NO₂ are detrimental inthat it gives a slower SCR reaction. For the LNT operation, it isnecessary to oxidize engine-out NO as fully as possible to NO₂ as LNT'sabsorb NOx principally as nitrates. Tailoring the zoned-CSF's catalystloading and ratio would achieve this.

Suitable SCR catalyst compositions for use in the system are able toeffectively catalyze the reduction of the NOx component at temperaturesbelow 600° C., so that adequate NOx levels can be treated even underconditions of low load which typically are associated with lower exhausttemperatures. Preferably, the catalyst article is capable of convertingat least 50% of the NOx component to N₂, depending on the amount ofreductant added to the system. Useful SCR catalyst compositions used inthe inventive system also have should resist degradation upon exposureto sulfur components, which are often present in diesel exhaust gascompositions.

The NOx reducing catalyst may comprise a lean NOx catalyst. Lean NOxcatalysts are typically classified as either a low temperature NOxcatalyst or a high temperature NOx catalyst. The low-temperature leanNOx catalyst is platinum based (Pt-based) and does not have to have azeolite present to be active, but Pt/zeolite catalysts appear to havebetter selectivity against formation of N₂O as a by-product than othercatalysts, such as Pt/alumina catalysts. Generally, a low temperaturelean NOx catalyst has catalytically active temperature ranges of about180 to 350° C. with highest efficiencies at a temperature of about 250°C. High temperature lean NOx catalysts have base metal/zeolitecompositions, for example Cu/ZSM-5. High temperature NOx catalysts havea lower temperature range of about 300-350° C. with highest efficiencyoccurring around 400° C. Different embodiments of the present inventionuse either high or low temperature lean NOx catalysts with an HCreductant.

The NOx reducing catalyst may comprise a lean NOx trap. In general, alean NOx trap containing a combination of a NOx sorbent and an oxidationcatalyst, which sorbs NOx onto the trap member during selected periodsof time, e.g., when the temperature of the gaseous stream is not suitedfor catalytic lean NOx abatement. During other periods of time, e.g.,when the temperature of the gaseous stream being treated is suitable forcatalytic lean NOx abatement, the combustible component on the trap isoxidized to thermally desorb the NOx from the trap member. A lean NOxtrap typically comprises a catalytic metal component such as one or moreplatinum group metals and/or a base metal catalytic metal component suchas oxides of one or more of copper, cobalt, vanadium, iron, manganese,etc.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrases such as “in one ormore embodiments,” “in certain embodiments,” “in one embodiment” or “inan embodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It will be apparent to those skilled in the art thatvarious modifications and variations can be made to the method andapparatus of the present invention without departing from the spirit andscope of the invention. Thus, it is intended that the present inventioninclude modifications and variations that are within the scope of theappended claims and their equivalents.

1. An emission treatment system for treating an exhaust streamcontaining nitrogen oxides and particulate matter comprising: aflow-through oxidation catalyst; a reductant injector for introducing areductant into the exhaust stream located downstream from the oxidationcatalyst; a particulate filter that does not contain an SCR catalyst forremoving the particulate matter from the exhaust stream locateddownstream from the reductant injector; and an SCR catalyst forpromoting chemical reactions between the nitrogen oxides and thereductant located downstream from the particulate filter, theparticulate filter being effective to disperse the reductant prior tothe reductant contacting the SCR catalyst.
 2. The emission treatmentsystem of claim 1, wherein the particulate filter does not contain anycatalyst.
 3. The emission treatment system of claim 1, wherein theparticulate filter and the SCR catalyst are closely coupled in the samecanister.
 4. The system of claim 1, further comprising an ammoniaoxidation catalyst located downstream from the SCR catalyst.
 5. Theemission treatment system of claim 1, wherein the particulate filter iscatalyzed.
 6. The emission treatment system of claim 1, wherein thereductant is urea and the particulate filter is configured so thatsubstantially all of the urea that enters the particulate filter isconverted to ammonia and isocyanic acid when it exits.
 7. The emissiontreatment system of claim 1, further comprising a hydrocarbon injectorfor introducing hydrocarbon compounds into the exhaust stream locatedupstream of the oxidation catalyst.
 8. The emission treatment system ofclaim 7, wherein the system further includes a metering system in fluidcommunication with the reductant injector.
 9. The emission treatmentsystem of claim 6, wherein the reductant comprises urea and water. 10.The emission treatment system of claim 1, wherein the SCR catalyst andthe ammonia oxidation catalyst are located on the same substrate. 11.The emission treatment system of claim 5, wherein the particulate filterhas a first catalyst zone for creating NO₂ from the oxidation of NO. 12.The emission treatment system of claim 11, wherein the particulatefilter further includes a second catalyst zone for the oxidation of CO.13. The emission treatment system of claim 12, wherein the particulatefilter further comprises a base metal oxide for burning soluble organicfraction of soot captured in the filter.
 14. The emission treatmentsystem of claim 13, wherein the particulate filter further comprises aurea hydrolysis catalyst.
 15. The emission treatment system of claim 1,wherein the ammonia oxidation catalyst includes a catalyst for thetreatment of CO and hydrocarbons.
 16. A method of treating an exhaustgas stream containing nitrogen oxides and particulate matter,comprising: passing the exhaust gas stream containing hydrocarboncompounds through an oxidation catalyst; injecting a reductant into theexhaust gas stream downstream from the oxidation catalyst and upstreamof a particulate filter that does not contain an SCR catalyst; passingthe exhaust gas stream through the particulate filter to cause thereductant to mix with the exhaust gas stream, the particulate filterbeing effective to disperse the reductant prior to the reductantcontacting the SCR catalyst and; passing the exhaust gas stream exitingthe particulate filter through an SCR catalyst separated from theparticulate filter.
 17. The method of claim 16, wherein the reductantcomprises urea and the exhaust gas exiting the particulate filtercontains substantially no urea.
 18. The method of claim 17, wherein theurea entering the exhaust gas filter is converted to ammonia andisocyanic acid prior to contacting the SCR filter.
 19. The method ofclaim 16, further comprising injecting hydrocarbon compounds into theexhaust gas stream upstream of the oxidation catalyst and passing theexhaust gas stream exiting the SCR catalyst through an ammonia oxidationcatalyst.
 20. The method of treating an exhaust gas stream of claim 16,wherein passing the exhaust gas stream through the particulate filtercomprises contacting the exhaust gas stream with a urea hydrolysiscatalyst on the filter.